Role of Fine Carbide Precipitates in the Grain Refining Behavior of 3.5%Ni-Cr-Mo-V Steel

Abstract

Grain refining behavior during austenitizing of 3.5%Ni-Cr-Mo-V turbine rotor shaft steel was investigated. Special attention was paid on the effect of Ni content and role of fine carbide precipitates on the austenite transformation and migration of the acicular austenite grain boundary. The microscopic observation of grain refining behavior reveals the following results.
(1) The grain refining of 3.5%Ni-Cr-Mo-V steels during austenitizing occurs through two stages. The first stage is austenite transformation which γ-phases succeed to the acicular bainitic ferrite structure. The second stage is the formation of equiaxed grain at higher temperatures by the migration of grain boundary of the acicular austenite.
(2) Driving force of the grain boundary migration in the second stage is considered to be the high density defects in the acicular austenite.
(3) Increase of Ni content stabilize the acicular γ-phases up to higher temperatures. This decreases the formation of equiaxed grain due to the migration of acicular austenite grain boundary.
(4) Increase of V content makes the grain refining difficult because the fine carbide precipitate such as VC works as a barrier against the grain boundary migration required for the formation of equiaxed grain.